Like a hand-held mower (shears) and a knapsack operation machine targeted to vert, conventional two-stroke and four-stroke small-size general engines have been widely used as engines of the operation machines that drive operating personnel to hold by hands or bear on the back for operation. Since contaminant discharge provisions become stricter, the need of changing a drive source from a two-stroke engine into a four-stroke engine is increasingly improved. However, the four-stroke engine has a more complicated lubricating system which causes the engine to help little when operating in a leant angle. Lubrication becomes the major design issue of each engine vendor. At present, only Honda produces four-stroke engines that can work under various leant angles according to EP835987 patent technology in mass production. According to the technology, a special oil tank is employed to surround a rotary blade installed on a crank shaft so as to produce oil mist. However, the engine designed in this manner is wider, heavier and more expensive. Moreover, American patent U.S. Pat. No. 6,213,079 of FUJI ROBIN discloses a lubricating system. However, this lubricating system is more complicated. Particularly, a complicated oilway channel is arranged on a cylinder block, so that the processing is complicated and the cost is improved.